Lignin peroxidase-type activity of soybean peroxidase

Soybean peroxidase (SBP), an acidic peroxidase isolated from the hulls of the bean, catalyzes the efficient oxidation of veratryl alcohol to veratraldehyde in the presence of H₂O₂. The reaction is optimal at pH 2.4 in the presence of 0.2 m CaCl₂. Soybean peroxidase is highly thermostable at pH 2.4, with half-lives of 210 and 2.5 h at 30 and 50°C, respectively. This compares favorably to the thermostability of lignin peroxidase (LiP) from Phanerochaete chrysosporium under equally acidic conditions. In fact, SBP is at least 150-fold more stable than LiP at 30°C and the latter is completely labile at 50°C. Soybean peroxidase follows a ping-pong, bi-bi catalytic reaction mechanism with a k_cat K_m (veratryl alcohol) of2.47 · 10² m^-1s^-1, ca. 1500-fold lower than a similar value for lignin peroxidase. This lower value of catalytic efficiency is due both to a higher K_m (veratryl alcohol) and lower k_cat for SBP as compared to LiP. Oxidation of methoxybenzenes suggests that the approximate oxidation potential of SBP is 1.42 V, yet this is high enough to effect the oxidation (and eventual β-ether cleavage) of 1-(3,4-dimethoxyphenyl)-2-(phenoxy)propan-1,3-diol, a lignin model dimer. In addition to SBP, horseradish peroxidase (HRP) is also capable of oxidizing veratryl alcohol as well as methoxybenzenes (the latter up to an oxidation potential of 1.34 V). Horseradish peroxidase, however, is extremely labile at pH 2.4 and is inactivated within minutes under such acidic conditions. The oxidization bySBP, and to a lesser extent by HRP, of veratryl alcohol directly in the presence of H₂O₂ is the first reported case of plant peroxidases catalyzing the efficient oxidation of high oxidation potential nonphenolics and demonstrates that SBP may be an effective alternative to lignin peroxidase.